1. Field of the Invention
The present invention relates to a system for controlling the operation of a microwave oven and determining the cooking time for a food item, and more particularly to a system for monitoring the change in color of a thermochromic element associated with a microwave food package.
2. Description of the Related Art
Owners of microwave ovens often find it difficult to assess whether food is properly cooked. It is hard to know whether the inside of food is hot enough—or overcooked—even when correctly using guidelines on timing. Because the air inside a microwave oven does not heat up significantly compared to the food or packaging being heated, it is difficult to measure temperature of food within a microwave oven. The appearance of the food alone does not help to tell whether the inside of food is cooked or heated adequately because of the penetrative way in which the microwaves cook the food.
Many microwavable foods are purchased and supplied in packaging which can be directly placed inside a microwave and heated. In heating such packaged food items, it is likewise difficult to assess when the food is properly heated. In such a case, direct observation of the food is difficult or impossible to do and it is necessary to rely on guidelines for timing supplied with such food item. However, due to variations in a number of factors, such instructions are not always appropriate for proper heating. It is frequently the practice that heating or cooking of such food items is stopped and the oven door is opened so that the temperature of the food item may be tested, for example, by stirring or prodding the food item sometimes directly by finger. This is inconvenient and slows the cooking process, which can lead to a deterioration in the taste or texture of heated foods.
Various systems for directly sensing the temperature of food items heated in a microwave have been proposed. Published UK Patent Application GB 2 312 954 discloses a system of monitoring the temperature of a microwave container for heating food where the container is provided with a thermochromic composition which changes color over a desired temperature range and the color of the composition is monitored, visually or spectroscopically, during the heating process. In this way, it is possible to detect when the package or container has received sufficient microwave energy.
Swedish Patent SE 0302030 pertains to a device which provides temperature controlled heating of primarily solid food stuff in a microwave oven by monitoring the color changes in a heat sensitive, readable color (thermochromic) marking provided on a food stuff micro-wavable package. Since the temperature of the surface of the food stuff container during heating is not the same as the actual temperature of the food stuff, the color marking is located on the surface of a block of a polymer material, which is thermally insulated from the food stuff, and which is arranged to be subject to the same microwave radiation as the food stuff inside the oven, and which, through the choice of suitable dielectric properties, is subject to heating corresponding to that of the food stuff. The color change may be monitored by a detector.
While the prior proposed systems provide a partial solution to the problem of temperature measurement within a microwave oven, they do not address the practical realities of operating within a modern microwave oven. There is a need for a system which monitors the temperature of a food items being heated on a rotating microwave turn table.